This document discusses three key factors to consider when designing an electrical system for a datacenter: size, reliability, and complexity. It recommends calculating size based on floor area and power density or number of racks. Reliability is determined by the desired Tier level. Higher tiers require more redundant components and affect design. Complexity is influenced by size and reliability - more complex systems are more expensive. It provides examples of Cummins' work with datacenters, focusing on calculating needs based on these three factors.

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What you should know
before designing an
electrical system
Cummins suggests three key factors
to consider
Three factors have to be considered when
determining what the power-distribution
infrastructure will look like, writes Cummins’
senior manager of strategic accounts
Richard Hallahan. They are the size of this
infrastructure, the reliability architecture and
operational complexity.
Hallahan takes a deep dive on each of these
factors in his whitepaper titled Data center
design decisions and their impact on power
system infrastructure.
Size matters
Hallahan’s first crucial factor is the size of
the infrastructure. Definition of the power
infrastructure for the entire facility is
closely related to heat generated by the IT
equipment. This is because IT and cooling
infrastructure are the biggest energy
consumers in the data center and their peak
consumption defines the facility’s power
needs.
Hallahan recommends two methods for
calculating electrical consumption of the IT
equipment. The first method is multiplying
the data center floor’s heat load by its area.
Heat load in this equation is expressed in
watts per square foot. Thus, a 48,000 sq ft data
center floor with heat density of 150 watts per
sq ft will consume 7,200,000 watts, or 7.2MW.
The second method is multiplying heat load
of an IT rack by the total number of racks on
the floor at full capacity. Using this equation,
IT equipment in a 600-rack data center with
capacity at 12kW per rack will also consume
7.2MW total.
Hallahan suggests a simple rule of thumb for
using the IT power number to calculate an
approximate power the entire data center is
going to consume. Since mechanical systems
of a facility often consume about the same
amount of power as the IT gear on the raised
floor, total data center power can be arrived
at by simply multiplying IT power by two.
How reliable do you want
to go?
The second key factor, reliability architecture,
considers which Tier of the Uptime Institute’s
Tier Classification system for infrastructure
reliability the facility is going to be designed
to. The decision to build to one of the four
tiers will to a large extent dictate the power
infrastructure’s design. At Tier II, for example,
some (but not all) components of the power
system will be redundant.
The reliability architecture further defines
power system infrastructure. The same Tier II
data center will most likely have paralleling
switchgear to integrate both generators into
the infrastructure instead of using a transfer
switch.
The higher the chosen reliability tier, the more
expensive the system will be.
Another aspect that influences power system
architecture and the facility’s initial cost is the
choice of operating voltage. Since there is
no rule of thumb for determining what cost
a certain operating voltage will translate to,
Hallahan recommends comparing the cost of
a number of designs with different operating
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voltages to determine the optimal one.
The complexity factor
Hallahan’s third key factor that determines
design of the power infrastructure is the
system’s operational complexity. This factor is
closely tied to the second factor of reliability
level, he writes. A higher Tier for example,
will create the necessity to install paralleling
switchgear and other automatically
controlled devices. More equipment means
a more complex operational sequence for
reacting to changes in normal operating
conditions like loss of utility power.
When utility power is lost, the sequence is
designed to transfer the data center load to
redundant power-path interconnections,
which keep the facility running during
an outage. The amount of power paths
in different systems can be two or more,
depending on design complexity.
The level of operational complexity a power
distribution system is going to have will be
determined to a large extent by the two
choices described above: size and reliability.
Examples of common power configurations
are isolated bus, multiple transfer pairs, main-
tie-main split generator bus and more.
More complexity is added by integration
of paralleling, power-transfer and system
controls into the larger power distribution
system for the facility. This can include more
supervisory control via a SCADA (supervisory
control and data acquisition) system.
Hallahan says standard architectures are
usually more reliable than custom-designed
Staying power
Q&A with Richard Hallahan, senior
manager of strategic accounts,
Cummins. Author of Data center design
decisions and their impact on power
system infrastructure
DatacenterDynamics FOCUS: How
universal is the rule of thumb that a
facility’s total power consumption is its
IT load times two?
Richard Hallahan: It is just a“rule of
thumb” I was given years ago by a
senior engineer. It gives you an order
of magnitude from which to start when
you are at the conceptual design stage
of a project. It always seems that owners
know what power density (w/ft² or kw/
rack) they want to achieve when the
facility is completely built out very early
on in the project. As design evolves, the
owner and consultant should continue
to enter known power consumption
values of all the equipment into their
calculation to further refine sizing needs
of the backup power system.
DCDF: What do new-generation
intelligent infrastructure management
tools do to the way electrical
infrastructure is designed?
RH: The new tools have not had a
significant impact on infrastructure
design, but I expect they will in the
near future. Right now, these tools
can compile a considerable amount of
data about power consumption trends
within the facility. As facilities strive
to become more and more energy
efficient, trend analysis will become key
to understanding where opportunities
exist to improve energy efficiency.
Electrical infrastructure design will
evolve to take advantage of these
opportunities.
DCDF: What key factors are considered
in the decision about the redundancy
level of a data center infrastructure?
RH: Start by understanding the business
impact associated with a loss of power to
facility. Once the impact is understood
it will be up to the owner to decide
how robust they want the facility. This
will drive the quantity of components
needed to support the load.
DCDF: Where is energy efficiency on the
list of top-ten factors to consider when
designing an electrical infrastructure?
RH: In the last five years, energy
efficiency has moved into the top five
factors because the cost of electricity is
the single largest expense over the life
of a data center. At start of the design of
a facility, energy efficiency is probably
#5, but as the design evolves and the
higher factors, like meeting reliability
goals, maintainability of the system,
cost targets etc. are solved by the design
concepts, energy efficiency moves up
the list. Alternatives to improve energy
efficient will be proposed and evaluated
and most likely accepted since they
will positively affect the total cost of
ownership of the facility.
Cummins provided the power
system at the data center of
Phoenix NAP, a provider of
colocation and other data center
services, which include Cloud
offerings, in its namesake city.

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ones because they had been in operation
for many hours – enough to reveal flaws in
programming of sequences of operations.
Programming in standard architectures has
been used in multiple applications of the one
standard architecture, which makes it more
reliable than a custom design.
Another benefit of standard power system
architectures is easy replaceability of their
components. If any component of a standard
architecture needs to be replaced, the end
user usually has no trouble finding it on the
market. It should be readily available and
easily integrated into the infrastructure. This
reduces the potential for downtime.
Being able to easily replace failed parts is key
to equipment’s serviceability. More complex
custom-designed systems are also simply
more expensive. The more complex a system
is, the higher the cost of the hardware itself,
but there is also the added cost of testing
whether the system works as intended and
additional maintenance costs.
To summarize, a user has to know their future
data center’s floor area (or the number of racks)
and power density. They need to decide how
reliable of a facility they need and determine
the optimal level of complexity required.
Phoenix NAP’s focus on
reliability
Here’s an example of Cummins’power
architecture decision-making tenets applied
to a real-life project. Cummins provided the
power system at the data center of Phoenix
NAP, a provider of colocation and other
data center services, which include Cloud
offerings, in its namesake city. Cummins
delivered the data center’s entire standby
power system.
The top consideration for Phoenix NAP was
reliability, since its business success hinges on
its ability to ensure uptime for clients, which
include financial institutions, web properties
and web commerce companies.
At Phase I, the data center’s load would be 4MW,
expandable to 8MW in the second phase.The
power system had to be designed, however, to
eventually scale up to 40MW. Size of the data
center floor in Phase I was 45,000 sq ft.
The initial system was a digital paralleling
system that included DMC 200 controls, 10
section switchgear line-up and two 2MW
DQKAB generator sets in sound-attenuated
housing. More gen-sets would be added as
the business grows.
The system can be monitored and controlled
from three different locations in the data
center: control room, mechanical and
enclosure room.
Space at the site was limited, so finding
space to install UPS and generator systems
was a challenge. Cummins decided to put
the generator sets outdoors – hence the
acoustical enclosures. These enclosures (two
generators per enclosure) were custom-
designed. Paralleling gear was placed in an
outdoor enclosure as well.
Cox Communications’
complex needs
Another example of a real-life application of
Cummins’approach to power infrastructure
design was the work it did for Cox Enterprises.
Also in Phoenix, the service provider’s new
110,000 sq ft data center needed a standby
power system that would ensure the six
million residences and businesses in the
US that used Cox’s digital video, internet,
telephone and wireless services continued
receiving them uninterrupted.
Cummins installed a paralleling system with
two PowerCommand DCM 300 controllers
and three DQKAB diesel generator sets rated
at 2MW each. Cummins manufactured,
designed and integrated the system. It also
services it.
Cox needed 4.8MW of critical power, with
2.4MW of critical no-break power and 1.6MW
of short-break power for chillers, pumps,
air conditioners and lights. The initial phase
included three 2MW generators, but would
eventually have to accommodate three more.
Another requirement was for the system
to be able to shift load between A and B
switchgear, so total building load could be
supported by either of the two switchgear
sets for uninterrupted maintenance. n
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